function calculateMortgage() {
// Get Inputs
var homePrice = parseFloat(document.getElementById('mc_homePrice').value);
var downPayment = parseFloat(document.getElementById('mc_downPayment').value);
var interestRate = parseFloat(document.getElementById('mc_interestRate').value);
var loanTerm = parseFloat(document.getElementById('mc_loanTerm').value);
var propertyTax = parseFloat(document.getElementById('mc_propertyTax').value);
var homeInsurance = parseFloat(document.getElementById('mc_homeInsurance').value);
var hoaFees = parseFloat(document.getElementById('mc_hoaFees').value);
// Validation
if (isNaN(homePrice) || homePrice <= 0) {
alert("Please enter a valid Home Price.");
return;
}
if (isNaN(downPayment) || downPayment < 0) {
downPayment = 0;
}
if (isNaN(interestRate) || interestRate < 0) {
interestRate = 0;
}
if (isNaN(loanTerm) || loanTerm <= 0) {
alert("Please enter a valid Loan Term in years.");
return;
}
if (isNaN(propertyTax)) propertyTax = 0;
if (isNaN(homeInsurance)) homeInsurance = 0;
if (isNaN(hoaFees)) hoaFees = 0;
// Core Calculations
var loanAmount = homePrice – downPayment;
var monthlyRate = (interestRate / 100) / 12;
var numberOfPayments = loanTerm * 12;
var monthlyPrincipalInterest = 0;
// Handle 0% interest edge case
if (interestRate === 0) {
monthlyPrincipalInterest = loanAmount / numberOfPayments;
} else {
// Standard Mortgage Formula: M = P[r(1+r)^n]/[(1+r)^n-1]
var mathPower = Math.pow(1 + monthlyRate, numberOfPayments);
monthlyPrincipalInterest = loanAmount * (monthlyRate * mathPower) / (mathPower – 1);
}
var monthlyTax = propertyTax / 12;
var monthlyInsurance = homeInsurance / 12;
var totalMonthlyPayment = monthlyPrincipalInterest + monthlyTax + monthlyInsurance + hoaFees;
// Display Results
document.getElementById('res_principalInterest').innerHTML = "$" + monthlyPrincipalInterest.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('res_taxes').innerHTML = "$" + monthlyTax.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('res_insurance').innerHTML = "$" + monthlyInsurance.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('res_hoa').innerHTML = "$" + hoaFees.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('res_total').innerHTML = "$" + totalMonthlyPayment.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('res_loanAmount').innerHTML = "$" + loanAmount.toLocaleString('en-US', {minimumFractionDigits: 0, maximumFractionDigits: 0});
// Show result box
document.getElementById('mc_results').style.display = 'block';
}
Understanding Your Total Monthly Mortgage Payment (PITI)
When shopping for a home, most buyers focus strictly on the list price or the principal and interest payment. However, the actual amount withdrawn from your bank account every month is often significantly higher. This calculator determines your PITI—Principal, Interest, Taxes, and Insurance—along with Homeowners Association (HOA) fees to give you a realistic view of affordability.
What are the Components of a Mortgage Payment?
Principal: The portion of your payment that pays down the loan balance. In the early years of a 30-year mortgage, this is usually small compared to interest.
Interest: The cost of borrowing money. With higher rates (e.g., above 6%), interest can make up the majority of your payment for the first decade.
Property Taxes: Assessed by your local government. These are often bundled into your mortgage payment through an escrow account.
Homeowners Insurance: Protects your property against damage. Like taxes, this is usually paid monthly into an escrow account.
HOA Fees: If you buy a condo or a home in a planned community, you must pay mandatory fees for common area maintenance. These are typically paid directly to the association, not the lender, but they affect your debt-to-income ratio.
How Interest Rates Impact Your Buying Power
Small changes in interest rates can have a massive impact on your monthly payment. For a $400,000 loan, the difference between a 4% and a 7% interest rate is roughly $750 per month. This calculator allows you to stress-test your budget by adjusting the interest rate field to see how market fluctuations might affect your ability to purchase.
The 28/36 Rule of Affordability
Financial experts often cite the 28/36 rule for mortgage approval. This suggests that your housing expenses (PITI + HOA) should not exceed 28% of your gross monthly income, and your total debt service (housing + cars + credit cards) should not exceed 36%. Use the "Total Monthly Payment" result from the calculator above to ensure you stay within safe financial limits.
Tips for Lowering Your Monthly Payment
If the calculated payment is higher than your budget allows, consider the following strategies:
Increase Your Down Payment: Putting 20% down avoids Private Mortgage Insurance (PMI) and lowers the principal loan amount.
Shop for Insurance: Homeowners insurance premiums vary wildly. Getting quotes from multiple providers can save you $50-$100 a month.
Appeal Property Taxes: If your home's assessed value is higher than the market value, you can appeal your tax assessment to lower your yearly bill.